Random access memory (RAM) is a ubiquitous component of modern digital circuit architectures. RAM can be a standalone device, or can be integrated in a device that uses the RAM, such as a microprocessor, microcontroller, application specific integrated circuit (ASIC), system-on-chip (SoC), and other like devices. RAM can be volatile or non-volatile. Volatile RAM loses its stored information whenever power is removed. Non-volatile RAM can maintain its memory contents even when power is removed. Although non-volatile RAM has advantages, such as an ability to retain its contents without applied power, conventional non-volatile RAM has slower read/write times than volatile RAM.
Magnetoresistive Random Access Memory (MRAM) is a non-volatile memory technology having response (read/write) times comparable to volatile memory. Data stored in MRAM does not degrade over time and, compared to other RAM technologies, MRAM uses very little power. In contrast to conventional RAM technologies, which store data as electric charges or current flows, MRAM uses magnetic storage elements. As illustrated in FIGS. 1A and 1B, a magnetic tunnel junction (MTJ) storage element 100 can be formed from two magnetic layers—a fixed layer 110 and a free layer 130, each of which can retain a magnetic field and are separated by an insulating layer 120 (e.g., a tunnel barrier layer). One of the two layers (e.g., the fixed layer 110), is pinned to a particular polarity. The polarity of the other layer 140 (e.g., the free layer 130) is free to change to match that of an externally-applied magnetic field. When the polarities of the fixed layer 110 and the free layer 130 are not aligned (i.e., antiparallel), the MTJ storage element 100 has a higher electrical resistance than when the polarities of the fixed and free layers are aligned (i.e., parallel). Thus, a change in the polarity 140 of the free layer 130 changes the resistance of the MTJ storage element 100. For example, when the polarities are aligned, as depicted in FIG. 1A, the MTJ storage element 100 has a relatively low electrical resistance. When the polarities are not aligned, as depicted in FIG. 1B, then the MTJ storage element 100 has a relatively high electrical resistance. Therefore, the MTJ storage element 100 can represent logic “0” in one of these magnetic states and to represent logic “1” in the other state, thus allowing the MTJ storage element 100 to be used as a magnetic memory element in an MRAM. The depiction of MTJ storage element 100 in FIGS. 1A and 1B is simplified, and each depicted layer can comprise one or more layers of materials.
Referring to FIG. 2A, a conventional memory cell 200 of a conventional field switching MRAM is depicted during a read operation. The memory cell 200 includes a transistor 210, a bit line 220, a digit line 230, and a word line 240. The memory cell 200 is read by measuring the electrical resistance of the MTJ storage element 100. For example, the MTJ storage element 100 in the memory cell 200 can be selected from a group of MTJ storage elements by activating an associated transistor 210 to switch current from a bit line 220 through the MTJ storage element 100. Due to a tunnel magnetoresistive effect, the electrical resistance of the MTJ 100 is based on the relative orientation of the polarities of the two magnetic layers (e.g., the fixed layer 110, the free layer 130). For example, if the fixed layer 110 and the free layer 130 have the same polarity, the resistance is low and a first logic state (e.g., a logic “0”) is read. If the fixed layer 110 and the free layer 130 have opposing polarities, the resistance is higher and a second logic state (e.g., a logic “1”) is read. When a current is passed through the MTJ storage element 100, a voltage drop is created across the MTJ storage element 100 due to the electrical resistance of the MTJ storage element 100. The voltage drop across the MTJ storage element 100 is compared to a reference voltage (e.g., a reference bit line) to determine if the resistance of the MTJ storage element 100 is relatively high or low, thus determining if the MTJ storage element 100 is in the first logic state or the second logic state.
Referring to FIG. 2B, the memory cell 200 of a conventional field switching MRAM is depicted during a write operation, which is a magnetic operation. Transistor 210 is off during the write operation. Current flows through the bit line 220 and the digit line 230 to establish magnetic fields 250 and 260, which affect the polarity of the free layer 130 of the MTJ storage element 100, and consequently the logic state of the memory cell 200. Accordingly, data is written to, and stored in, the MTJ storage element 100 and thus the memory cell 200.
MRAM has several desirable characteristics that make it a candidate for a universal memory, such as high speed, high density (i.e., small bitcell size), low power consumption, and no logic state degradation over time. Accordingly, a non-volatile MRAM memory can be fabricated from an array of the memory cells 200.
Despite the characteristics described above, conventional MRAM devices are not perfect. Due in part to fabrication variations, resistances of MTJs in a reference bit line can vary from MTJ to MTJ. This leads to reference voltage variations, poor sensing margin, and read disturbances.
Conventional attempts to resolve these problems include a conventional merged reference generator device 300, as depicted in FIGS. 3A-3C. FIG. 4 also depicts a conventional approach using a conventional reference bitline 400. These approaches can suffer from excessive complexity, can require additional integrated circuit layers, and can ineffectively mitigate the read disturbances they are intended to resolve.
Accordingly, there are long-felt industry needs for methods and apparatus that improve upon conventional methods and apparatus, including the improved methods and apparatus provided hereby.